1. Field of the Invention
The present invention relates to a semiconductor display device using thin-film transistors. In particular, the invention relates to a semiconductor display device in which a pixel switching circuit and driver circuits are formed on the same substrate in an integral manner.
2. Description of the Related Art
In recent years, the techniques of forming semiconductor devices, such as thin-film transistors (TFTs), by using a semiconductor thin film formed on an inexpensive glass substrate have made rapid progress. This is because of increased demand for active matrix liquid crystal display devices.
In active matrix liquid crystal display devices, TFTs are provided for respective ones of hundreds of thousands to millions of pixel regions that are arranged in matrix and charge that enters or exits from each pixel electrode is controlled by the switching function of the associated TFT.
The basic configuration of an active matrix liquid crystal display device in which thin-film transistors are arranged will be described below with reference to FIGS. 34A and 34B. FIG. 34A is a sectional view obtained by cutting a liquid crystal display device by a plane perpendicular to a substrate, specifically taken along a chain line Axe2x80x94Axe2x80x2 in FIG. 34B.
An insulating film (not shown) is formed on the surface of a transparent base substrate 1. Reference numeral 2 denotes an active layer of a TFT; 3, a gate electrode; 4, a data line; 5, a drain electrode; 6, an interlayer insulating film; 7, a black matrix; 8, a transparent conductive film as a pixel electrode; and 9, an alignment film.
In this specification, the structure including the base substrate 1 and the other members mentioned above (including the TFTs) is called an xe2x80x9cTFT substrate.xe2x80x9d Although FIG. 34A focuses on a single pixel, actually the TFT substrate is composed of a pixel area including hundreds of thousands to millions of pixel switching TFTs (called pixel TFTs) and peripheral driver circuit areas including a number of TFTs for driving the pixel TFTs.
On the other hand, reference numerals 10-12 denote a transparent substrate, a transparent conductive film as an opposed electrode, and an alignment film, respectively. The structure including these members, which is opposed to the TFT substrate, is called an xe2x80x9copposed substrate.xe2x80x9d
As shown in FIG. 35A, the TFT substrate 20 and the opposed substrate 30 are subjected to an alignment treatment such as rubbing for giving proper alignment to a liquid crystal. Thereafter, to control a substrate interval (cell gap) between the TFT substrate 20 and the opposed substrate 30, grainy spacers 41 are uniformly scattered over the entire surface of the TFT substrate 20. Then, a sealing agent 42 is printed. The sealing agent 42 has a role of an adhesive for bonding the substrates 20 and 30 together as well as a role of a sealing material for sealing the space between the substrates 20 and 30 to prevent a liquid crystal material that will be injected there from leaking to the outside of the substrates.
FIG. 36 is a sectional view of the TFT substrate 20. Since the grainy spacers 41 are uniformly scattered over the entire surface of the TFT substrate 20 to control the cell gap, the spacers 41 exist in not only the pixel area 22 but also the peripheral driver circuit regions 23 as shown in FIG. 36. Usually, the pixel TFTs formed in the pixel area 22 are not much different in device size from the driver circuit TFTs formed in the driver circuit areas 23. However, the black matrix for covering the pixel TFTs, the pixel electrodes that are transparent conductive films, and other members are formed in the pixel area 22. Further, in reflection-type liquid crystal display devices, a reflective electrode is formed in the pixel area 22. On the other hand, connection lines necessary to constitute CMOS circuits for driving the pixel TFTs are formed in the driver circuit areas 23. Therefore, there are differences in the height (distance) from the surface of the base substrate 1 between the pixel area 22 and the driver circuit areas 23.
A description will now be made of a case where the height as measured from the surface of the substrate 1 in the pixel area 11 is greater than in the driver circuit areas 23. The grainy spacers 41 are scattered in not only the pixel area 22 but also the driver circuit areas 23 by a wet or dry method. If the grainy spacers 41 have approximately uniform sizes, they have differences in the height as measured from the substrate 1 depending on their positions. Now, the height of the top of each spacer 41 in the pixel area 22 and that of the top of each spacer 41 in the driver circuit areas 23 are represented by hp and hd, respectively. As seen from FIG. 36, a height difference xcex94h=hpxe2x88x92hd occurs due to the difference in height between the pixel area 22 and the driver circuit areas 23.
Then, as shown in FIG. 37A, the TFT substrate 20 and the opposed substrate 30 are bonded together with the sealing agent 42. Thereafter, the space between the TFT substrate 20 and the opposed substrate 30 are filled with a liquid crystal material 43 and a liquid crystal injection inlet 44 is sealed with a sealing material (see FIG. 37B). In this manner, an active matrix liquid crystal display device having the configuration shown in FIG. 34A is obtained.
However, the liquid crystal display device having the above configuration has the following problems.
Because of the height difference xcex94h that is caused by the difference in height between the pixel area 22 and the driver circuit areas 23, the cell gas cannot be made uniform, that is, a cell thickness variation occurs, when the TFT substrate 20 and the opposed substrate 30 are bonded together. Further, as shown in FIGS. 37A and 37B, strain occurs in the opposed substrate 30. Defects such as display unevenness and an interference fringe (on the top surface of the opposed substrate) may occur in a liquid crystal display device having a cell thickness variation and strain in the opposed substrate 30.
Where the height as measured from the substrate 1 in the driver circuit areas 23 is greater than in the pixel area 22, because of the above-described height difference xcex94h, unduly strong force is exerted on the spacers 41 that are scattered in the driver circuit areas 23 when the TFT substrate 20 and the opposed substrate 30 are bonded together. As a result, the driver circuit TFTs having a more complex structure than the pixel TFTs are damaged considerably, which adversely affects the yield of products.
Where grainy spacers 15 exist in the pixel area, disorder in image display (disclination) may be observed as shown in FIG. 34B because the alignment of the liquid crystal material is disordered in the vicinity of the spacers 15.
As described above, where the cell gap is controlled by using conventional grainy spacers, satisfactory display may not be obtained due to various factors.
In liquid crystal display devices that are commonly manufactured or manufactured as trial products, the cell gap appears to be set at 4-6 xcexcm irrespective of the pixel pitch. However, in the future, liquid crystal panels will be required to have higher resolution and hence the pixel pitch will be increasingly reduced.
For example, projection-type liquid crystal display devices are desired to be able to display images having as high resolution as possible in view of the fact that the images are projected onto a screen in an enlarged manner. Also from the viewpoint of the cost, the optical system needs to be miniaturized and the panel size needs to be reduced. For the above reasons, in the future, it will be necessary to manufacture liquid crystal display devices having a pixel pitch of 40 xcexcm or less, preferably 30 xcexcm or less.
In liquid crystal display devices for displaying such high resolution images, even grainy spacers of several micrometers in diameter may deteriorate display quality when they exist in the effective display area.
Further, when a liquid crystal material is injected, the flow of the liquid crystal material forces conventional grainy spacers themselves to flow. As a result, a uniform spacer dispersion density profile may not be obtained, to cause a cell thickness variation.
Because of their characteristics, liquid crystal display devices using a ferroelectric liquid crystal that attract much attention recently and reflection-type liquid crystal display devices are required to have small cell gaps.
However, with conventional grainy spacers, it is generally difficult to produce a cell having a small, uniform-profile cell gap.
An object of the present invention is to provide a semiconductor display device that is free of a cell thickness variation and display unevenness by producing a cell having a small, uniform-profile cell gap that is hard to realize with conventional grainy spacers.
Another object of the invention is to prevent TFTs from being damaged by preventing unnecessary stress that would otherwise be exerted on the TFTs in bonding substrates together when conventional grainy spacers are used.
According to one aspect of the invention, there is provided an electro-optical device comprising a first substrate comprising a pixel area having a plurality of thin-film transistors and a plurality of pixel electrodes electrically connected to the respective thin-film transistors; a driver circuit area provided at a location separate from the pixel area and having a plurality of driver circuits having a plurality of thin-film transistors for driving the thin-film transistors in the pixel area; and a base substrate; a second substrate that confronts the first substrate; a plurality of gap retaining members; and a sealing member for bonding the first and second substrates together, wherein a distance from a surface of the base substrate to a surface of the pixel area is different from a distance from the surface of the base substrate to a surface of the driver circuit area and wherein the gap retaining members are formed in an area other than the pixel area and the driver circuit area. The above objects can be attained by this electro-optical device.
According to another aspect of the invention, there is provided an electro-optical device comprising a TFT substrate comprising a pixel area having a plurality of pixel electrodes arranged in matrix form and a plurality of pixel thin-film transistors electrically connected to the respective pixel electrodes; a driver circuit area having a driver circuit having a plurality of thin-film transistors for driving the pixel thin-film transistors; and a base substrate; an opposed substrate that confronts the TFT substrate; a display medium held between the TFT substrate and the opposed substrate, an optical response of the display medium being controlled by an application voltage; and a plurality of gap retaining members, wherein a distance from a surface of the base substrate to a surface of the pixel area is different from a distance from the surface of the base substrate to a surface of the driver circuit area and wherein the gap retaining members are formed in an area other than the pixel area and the driver circuit area. The above objects can be attained by this electro-optical device.
The above-mentioned optical medium may be such that its optical characteristic is modulated in accordance with the application voltage.
The above-mentioned display medium may be a liquid crystal.
The above-mentioned display medium may be a mixed layer of a liquid crystal material and a polymer.
The above-mentioned display medium may be an electroluminescence element.
The above-mentioned gap retaining members may be formed around the pixel area.
The arrangement density of the above-mentioned gap retaining members may be uniform in the pixel area.
Each of the above-mentioned gap retaining members may be shaped like a cylinder.
Each of the above-mentioned gap retaining members may be shaped like an elliptical pole.
Each of the above-mentioned gap retaining members may be shaped like a polygonal prism.
Each of the above-mentioned gap retaining members may be shaped so as not to obstruct a flow of the liquid crystal when it is injected.
The side face of each of the above-mentioned gap retaining members may be tapered.
The above-mentioned gap retaining members may be made of one material selected from the group consisting of polyimide, acrylic, polyamide, and polyimideamide.
The above-mentioned gap retaining members may be made of an ultraviolet curable resin.
The above-mentioned gap retaining members may be made of an epoxy resin.
According to another aspect of the invention, the top surfaces of the respective gap retaining members on the side of one of the first and second substrates have been planarized by chemical mechanical polishing. In this electro-optical device, since the cell gap is controlled by planarizing the top surfaces of the gap retaining members, a small cell thickness having a uniform profile over the entire electro-optical device can be obtained. Even if the gap retaining members are formed on the pixel area or the driver circuits, a uniform cell thickness profile can be obtained.
To attain the above objects, according to another aspect of the invention, there is provided an electro-optical device comprising a first substrate comprising a pixel area having a plurality of pixel electrodes and switching elements connected to the respective pixel electrodes; a second substrate confronting the first substrate; and a gap retaining member that is provided on the second substrate and retains an interval between the first and second substrates.
To attain the above objects, according to a further aspect of the invention, there is provided an electro-optical device comprising a first substrate comprising a pixel area having a plurality of pixel electrodes and switching elements connected to the respective pixel electrodes; a second substrate confronting the first substrate; a liquid crystal sealed in a space between the first and second substrates; a first alignment film that is formed on a surface of the first substrate and orients a liquid crystal; a second alignment film that is formed on the second substrate and orients the liquid crystal; and a gap retaining member that is provided on the second substrate and retains an interval between the first and second substrates.
In the above two electro-optical devices, the use of the gap retaining members provides the following advantages. First, it is not longer necessary to use spacers. Second, since the height of the gap retaining members can be set as desired, the interval between the substrates can be determined as desired. Third, since the gap retaining members are fixed, they are not gathered unlike the conventional spacers. Therefore, point defects do not occur.
In the above two electro-optical devices, the position of the gap retaining members can be set as desired. For example, the gap retaining members can be provided in an area that substantially confronts the pixel area. In this case, it is preferable that the gap retaining members be provided at locations that are not used for display, for instance, on a black matrix of color filters and bus lines in the pixel area. Alternatively, by providing the gap retaining members in an area that does not confront the pixel area, the interval between the substrates can be retained without causing no influences on the display.
Where the invention is applied to an electro-optical device in which a first substrate (TFT substrate) is provided with a pixel area and a driver circuit area having driver circuits for driving switching elements that are provided in the pixel area, it is preferable that the gap retaining members be provided on a second substrate (opposed substrate) in an area that does not confront the driver circuit area. In this case, it is possible to prevent the driver circuits from being damaged or destroyed by stress that is imposed by the gap retaining members.
According to the invention, since the gap retaining members are provided on the second substrate, influences (solvent or etchant-related influences, mechanical impact, etc.) of the formation of the gap retaining members do not affect the first substrate. Provided with the pixel area and the driver circuits, the first substrate has a much higher integration density than the second substrate. In view of this, the gap retaining members are provided on the second substrate to minimize the number of processes that are executed on the first substrate.
Further, by providing the gap retaining members on the second substrate, the conditions that are set in selecting a material are relaxed. For example, where the invention is applied to a TFT-type liquid crystal display device, since pixel TFTs and driver circuit TFTs are formed in the first substrate (TFT substrate), it is necessary to select a material capable of providing a sufficiently large selective etching ratio to the material of those TFTs.
On the other hand, only such members as an opposed electrode and color filters are formed on the second substrate (opposed substrate), that is, the number of materials used in the second substrate is smaller than in the first substrate. Thus, the number of conditions to be set in selecting a material is small. Further, a material such as that of an etching liquid, an etching gas, or the like and means that are necessary to form the gap retaining members can be selected from wider ranges.
To make it possible to uniformize the interval between the substrates, it is preferable that the gap retaining members be made of a planarization material capable of canceling out the asperity of the base member. For example, the gap retaining members may be made of a resin material selected from polyimide, acrylic, polyamide, and polyimideamide, an ultraviolet curable resin, or a thermosetting resin as typified by an epoxy resin.
The above resin materials are frequently used as interlayer insulating films of the TFT substrate (first substrate). In such a case, it is difficult to provide a large selective etching ratio if the gap retaining members made of a resin material are provided on the TFT substrate. This is the reason why in the invention the gap retaining members are formed on the opposed substrate (second substrate).